Processes and Device for Low Temperature Separation of Air

ABSTRACT

The process and device serve for the low-temperature separation of air in a distillation-column system includes at least one separation column. A main air stream is compressed in an air compressor at a first pressure and is then purified. A first air stream, which is formed from at least one part of the purified main air stream, is further compressed at a second pressure that is higher than the first pressure. From the further compressed first air stream, a throttling stream and a turbine stream are branched off. The throttling stream is cooled down and liquefied or pseudo-liquefied in a main heat exchanger and is then passed on to expansion equipment. The expanded throttling stream is conducted into the distillation-column system. The turbine stream is cooled down in the main heat exchanger and under an intermediate temperature of the main heat exchanger is conducted into an expansion machine.

This U.S. patent application claims priority of German patent documentDE 10 2009 042410.5 filed on Sep. 21, 2009 and German patent document DE10 2009 0484156.6 filed on Oct. 7, 2009, the entireties of which areincorporated herein by reference.

FIELD OF INVENTION

The invention is directed to a method and device for the low temperatureseparation of air with a distillation-column system.

BACKGROUND OF INVENTION

A process in which a product flow of liquid at pressure is vaporizedagainst a heat carrier and is finally obtained as a gaseous compressedproduct is also called an internal compression process. The process isparticularly widespread for obtaining compressed oxygen, but can also beused to obtain compressed nitrogen or compressed argon. For the case ofa supercritical pressure in a main heat exchanger, no phasetransformation occurs in a real sense; the product stream is then“pseudo-vaporized”.

Compared to the (pseudo-)vaporized product stream, a heat carrier underhigh pressure is liquefied in the main heat exchanger (orpseudo-liquefied if it is under supercritical pressure), namely afractional stream of air, which here is called a “throttling stream”.

It is customary to bring a throttling stream and a turbine streamtogether in a recompressor or in the main air compressor at a higherpressure, as is required for the distillation. This pressure must besufficiently high for the vaporization or pseudo-vaporization of theproduct stream made liquid at pressure and can be, for example, 20 or 60bars. The turbine stream is then, of course, also expanded at thispressure (“second pressure”) at roughly the operating pressure of thehigh-pressure column. Alternatively, the throttling stream is furthercompressed at a still higher pressure (“third pressure”).

The turbine stream serves initially for refrigeration. But in systemswith internal compression, it has a second function. The turbine streamhelps the throttling stream to evaporate (or to pseudo-evaporate) aninternally-compressed stream (nitrogen, oxygen, and/or argon). Thelarger the turbine stream, and the more this stream is cooled down inthe main heat exchanger (the greater the temperature difference betweeninlet and outlet), the more heat is made available for the (pseudo-)evaporation of the internally-compressed product stream, and the smallerthe throttling stream. The average temperature difference in the heatexchanger is thereby smaller, the temperature profile more favorable,and the system more efficient. This means it is always advantageous tocool the turbine stream down in the heat exchanger as much as possible.This generally leads to the stream at the turbine outlet not beinggaseous, but in fact partially liquefied.

Lowering the temperature at the turbine inlet is however notunconditionally possible but, with the machines generally used, amaximum liquid fraction of roughly 6% to a maximum of 10% (designcriterion) is provided. Higher liquid fractions can lead to turbinedamage. The inlet temperature in an air turbine is limited by thisrestriction, for example, with 60 bars at the inlet and roughly 85%efficiency at about 169 K. For an inlet pressure of 20 bars, thesmallest possible turbine inlet temperature is roughly 125 K. A goal isto set the turbine inlet temperature lower without violating the turbinedesign criterion, resulting in a more efficient process.

SUMMARY OF INVENTION

The present invention is based on the problem of achieving anenergy-efficient process and a corresponding device, with acomparatively low equipment cost.

This problem is resolved by the present invention. The turbine stream isno longer taken off in the operation of cooldown from an intermediateposition of the main heat exchanger, but passes further through the mainheat exchanger, so that the turbine stream, at subcritical pressure upto roughly the dewpoint temperature, is either cooled down more or, atsupercritical pressure, is pseudo-liquefied. Finally, the stream isexpanded in the main heat exchanger at an intermediate pressureoptimized with respect to expansion to produce work and to thetemperature profile. The stream is preferably expanded with a throttlevalve, and is heated up again in the main heat exchanger at theintermediate temperature, which corresponds to the inlet temperature ofexpansion to produce work and is as low as possible, so that the turbinedesign criterion is not violated. This intermediate temperature lies,for example, below 169 K for a 60-bar turbine stream or below 125 K fora 20-bar turbine stream.

The cooldown and (pseudo-)liquefaction of the turbine stream in the mainheat exchanger can then, if its pressure is equal to that of thethrottling stream, occur along with the throttling stream or separatelyfrom it. The intermediate pressure, at which the turbine stream isexpanded before its expansion to produce work, is equal to or higherthan √{square root over(P_(throttling stream)·P_(high-pressure column)))}. That is, for a60-bar throttling stream, the intermediate pressure would lie at 18 barsor higher or for a 20-bar throttling stream at 10.5 bars (under theassumption that the pressure in the high-pressure column amounts to 5.5bars). Expansion at the intermediate pressure is preferably carried outin a throttle valve. Expansion to produce work is performed in anexpansion machine, which preferably is constructed as a turbine.

In a first variant of the present invention, a recompressor operates onexternal power; both the throttling stream and the turbine stream areunder the second pressure during cooldown in the main heat exchanger.Using a recompressor without intermediate offtake, the equipment costcan be kept low.

“Operates on external power” means that the corresponding compressor isnot operated by power self-produced in the air-separation process but,for example, by an electric motor, a steam turbine, or a gas turbine.

In a second variant of the present invention, the recompressor is drivenby an expansion machine, which is operated with a process stream of theprocedure, in particular by an expansion machine which is operated withthe turbine stream, in which the air compressor represents the onlymachine operated on external power for air compression.

The “only machine” is understood here to be a single-stage or multistagecompressor, whose stages are all connected to the same drive, in whichall the stages are put into the same housing or are connected to thesame gear system. In this second variant, the “first pressure” is abovethe highest pressure of the distillation-column system; in particular,it is clearly above the operating pressure of the high-pressure column.This pressure difference amounts to, for example, at least 4 bars and ispreferably between 6 and 16 bars. In this variant, the total air in theair compressor (except for possible smaller fractions such as, forexample, instrument air) is preferably completely divided up into thethrottling stream and the turbine stream.

The process stream, which is used to drive the recompressor, can insteadof the turbine stream be formed, for instance, by a third air stream,which is expanded at the operating pressure of the low-pressure column(Lachmann turbine) or by compressed nitrogen from thedistillation-column system, particularly from a high-pressure orlow-pressure column. The compressed nitrogen can, at the inlet into thecorresponding expansion machine, be almost at ambient temperature, or itis heated in front of an inlet into the expansion machine at atemperature above ambient (“hot gas expander”).

In both variants of the present invention, the throttling stream can beunder a higher pressure than the turbine stream. That is, the turbinestream during cooldown in the main heat exchanger is under the secondpressure and the throttling stream during cooldown in the main heatexchanger is under a third pressure, which is identical to the secondpressure or is higher than the second pressure.

For the further secondary compression from the second at pressure, asecond recompressor is installed in the second variant, which is drivenby an expansion machine that is operated with a process stream of theprocedure. Preferably, the recompressor that runs at the second pressureis driven by the expansion machine that is operated with the turbinestream, and the process stream that is used for driving the secondrecompressor is formed by a third air stream, which is expanded at theoperating pressure of the low-pressure column (Lachmann turbine) or bycompressed nitrogen from the distillation-column system, particularlyfrom a high-pressure or low-pressure column. Alternatively, the twodrives can be switched.

In a modification of the first variant, the present invention comprisesat least two stages instead of the recompressor and can also be drivenwith external power. The further compression at the second pressure thenoccurs in at least a first stage of the recompressor; the throttlingstream is further compressed downstream of the branching off of theturbine stream at least in the last stage of the recompressor at a thirdpressure, which is higher than the second pressure. The steps accordingto the invention of cooldown, expansion, and heat-up of the turbinestream create so much additional flexibility that the process can attaina high efficiency, even if the construction-dependent intermediateofftake pressures of the recompressors are in themselves unfavorable.

Preferably, the intermediate pressure is 1.5 to 5 bars below the secondpressure, that is, the turbine stream in front of the inlet into theexpansion machine is expanded by this pressure difference. Thisrelatively low throttling at the low temperature causes practically nopower loss and still allows the desired reduction in the inlettemperature of the expansion machine.

Preferably, the distillation-column system comprises a high-pressurecolumn and a low-pressure column, which is located above a maincondenser relative to heat exchange. The main condenser is constructedas a condenser-evaporator. The turbine stream is expanded in theexpansion machine preferably at roughly the operating pressure of thehigh-pressure column and is fed at least in part into the high-pressurecolumn.

A liquid oxygen stream, a liquid nitrogen stream, and/or a liquid argonstream can be used as the liquid product stream from thedistillation-column system. If more than one product is internallycompressed, many independent and of course appropriate types ofequipment for increasing pressure (as a rule, pumps or pairs of pumps)and independent paths must be provided through the main heat exchanger.

It is favorable if a second stream of air is formed out of anotherportion of the purified main air stream and the second stream of air iscooled down in the main heat exchanger under the first pressure and isconducted to the distillation-column system. This second stream of airis also called a direct air stream. Preferably, the main air stream,aside from a small portion of the instrument air used, if necessary, isdivided up into precisely the three parts cited here, namely the directair stream, the turbine stream, and the throttling stream.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention as well as further details of the invention are furtherclarified in the following with the aid of the embodiments schematicallyrepresented in the drawings.

FIG. 1 shows an embodiment of the first variant of the invention;

FIG. 2 shows a first embodiment of the second variant of the inventionwith a single turbine;

FIG. 3 shows a further embodiment of the second variant of the inventionwith two turbines;

FIG. 4 shows a further embodiment of the second variant of the inventionwith two turbines;

FIG. 5 shows a heat-exchange diagram (temperature relative to specificenthalpy) for a process according to prior art without throttling of theturbine stream; and

FIG. 6 shows a heat-exchange diagram for the process of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

In the embodiment of FIG. 1, the distillation-column system 50comprises, in the part which serves for the nitrogen-oxygen separation,a high-pressure column 14, a low-pressure column 15, and a maincondenser 16 constructed as a condenser-evaporator, at which the twocolumns stand relative to heat exchange.

Atmospheric air is sucked in as main air stream 1 to an air compressor2, and is brought there to a first pressure that corresponds roughly tothe operating pressure of the high-pressure column 14, is cooled down ina primary cooling 3 to roughly ambient temperature, and is conducted toan adsorptive air purification 4. A first portion of the purified mainair stream 5 is further compressed as a “first air stream” 6 in arecompressor 7 at a second pressure of at least 50 bars, for exampleroughly 60 bars. The high-pressure air 8 is conducted to the hot end ofa main heat exchanger 9 and is cooled down and pseudo-liquefied in themain heat exchanger. The pseudo-liquefied air is drawn off throughpiping 10 from the cold end of the main heat exchanger and is then splitup into a throttling stream 11 and a turbine stream 17. Conversely, thethrottled and turbine streams after the joint recompression 7 are alsocooled down and pseudo-liquefied together in the main heat exchanger.Alternatively, the turbine stream 17 could be taken off somewhat abovethe cold end of the main heat exchanger 9 (see FIG. 2.)

The throttling stream (“JT Air”) 11 is expanded in a throttle valve 12to roughly the operating pressure of the high-pressure column and isconducted through piping 13 in a liquid state, at least in part, intothe high-pressure column 14. Instead of the throttle valve 12, a fluidturbine can be installed. One portion 43 of the throttling stream can beimmediately drawn out again from the high-pressure column and aftercooldown 31 can be fed through piping 44 to the low-pressure column 15at an intermediate position.

The turbine stream 17, which is pseudo-liquefied along with throttlingstream, is expanded in a throttle valve 18 at an intermediate pressurebetween the operating pressure of the high-pressure column and thesecond pressure and is then conducted again to the cold end of the mainheat exchanger 9. In the main heat exchanger, it is again heated up toan intermediate temperature that is between 140 and 150 K. At thisintermediate temperature, the turbine stream is drawn through piping 70out of the main heat exchanger 9 and conducted to a turbine 19, which inthe example is slowed down by a generator 20. In the turbine 19, the airis expanded to produce work at roughly the operating pressure of thehigh-pressure column. The expanded turbine stream 21 is conducted into aseparator (phase separator) 22 in order to separate out liquidfractions, if necessary. Such liquid fractions 23 are fed in throughpiping 24 to a suitable location in the low-pressure column 15. Thegaseous fraction 25 is conducted through piping 26 as gaseous feed airinto the high-pressure column 14.

The remainder of the purified main air stream 5 is passed, withoutpressure-altering steps, through the main heat exchanger 9 as a directair stream (“second air stream”) 27, 28 and flows further through piping26 into the high-pressure column 14.

In a first version of the embodiment (system without argon yield), rawliquid oxygen 29 flows from the sump of the high-pressure column 14through piping 30, undercooling counterflow 31, and further throughpiping 32 to an intermediate position on the low-pressure column. Thegaseous nitrogen head 33 of the high-pressure column 14 is condensed atleast for the portion 34 in the liquefaction space of the main condenser16. Another portion can be passed over piping 35 through the main heatexchanger 9 and can finally be drawn off through piping 36 as a gaseousintermediate-pressure product (PGAN).

The condensed nitrogen 37 is delivered from the main condenser 16 to afirst portion 38 as a return flow at the high-pressure column 14. Asecond portion 39 is cooled down in the undercooling counterflow 31 andpassed through piping 40 to the low-pressure column 15 as return flow.

Likewise, a nitrogen-enriched stream 41, 42 can be conducted from anintermediate position on the high-pressure column 14 through theundercooling counterflow 31 to an intermediate position on thelow-pressure column 15.

From the sump of the low-pressure column, a low-pressure gaseous oxygenproduct 45 (GOX) can be directly taken off, heated in the main heatexchanger 9, and be drawn off through piping 46 as a low-pressureproduct.

The oxygen desired as a gaseous compressed product is drawn off as aliquid (LOX) out of the low-pressure column or out of the evaporationspace of the main condenser 16 and passes as a “first liquid productstream 47 of internal compression (IC-LOX). Here it is brought in aliquid state by an oxygen pump 48 to the desired increased pressure(first increased pressure) and conducted through piping 49 to the coldend of the main heat exchanger 9. In the main heat exchanger 9, theliquid oxygen stream 49 is vaporized or pseudo-vaporized under theincreased pressure and heated up to roughly ambient temperature. Itfinally leaves the system through piping 51 as a first gaseouscompressed product (HP-GOX).

If desired, a further gaseous oxygen product 53, 54 (MP-GOX) can beobtained under an intermediate pressure that is between the operatingpressure of the low-pressure column 15 and the increased pressuredownstream of the pump 48, in which this fraction branches offdownstream of the pump 48, is appropriately throttled down 52, and isfinally vaporized and heated up separately in the main heat exchanger 9.

Alternatively, or in addition to the internally compressed oxygen streamor streams, nitrogen can be passed on for internal compression. What ismore, a third portion 55 of the condensed nitrogen 37 is brought as asecond “liquid product stream” out of the main condenser 16 (HP-LIN)into a nitrogen pump 56 at a second increased pressure that correspondsto the desired product pressure and which must not be the same as thefirst increased pressure. The high-pressure nitrogen is conductedthrough piping 57, 58 to the cold end of the main heat exchanger 9. Inthe main heat exchanger 9, the liquid or supercritical nitrogen stream58 is vaporized or pseudo-vaporized under the increased pressure and isheated up to roughly ambient temperature. It finally leaves the systemthrough piping 59 as a second gaseous compressed product (HP-GAN).

If desired, a further gaseous nitrogen product 61, 62 (MP-GAN) can beobtained under an intermediate pressure that is between the operatingpressure of the high-pressure column 16 and the increased pressuredownstream of the pump 56, in which this portion branches off downstreamof the pump 56, accordingly throttled down 60, and finally vaporized andheated up separately in the main heat exchanger 9.

As further return flows, unpurified nitrogen 63, 64, 65 and unpurifiednitrogen 66, 67, 68 are drawn gaseous out of the low-pressure column 15into the undercooling counterflow 31 and are further heated up in themain heat exchanger 9 and drawn off as low-pressure products (GAN, UN2).Finally, a portion of the products are also obtained as liquid, forexample liquid nitrogen (LIN) 69 or a portion of the liquid oxygen (LOX)47 from the sump of the low-pressure column 15.

The process of the embodiment of the first version can, for example,also be operated with only one liquid product stream and one gaseouscompressed product (for instance, either oxygen or nitrogen), oralternatively with any combination of the streams depicted 49, 53, 58,and 61 made liquid at pressure.

In an embodiment of the second version, the distillation-column systemof the embodiment additionally exhibits an argon fraction 100 for theequipment for nitrogen-oxygen separation, which serves to yield pureliquid argon (LAR) 105. The argon fraction comprises one or moreraw-argon columns for argon-oxygen separation and a pure-argon columnfor argon-nitrogen separation, which is operated in the known manner.The lower end of the raw-argon column communicates through the piping101 and 102 with an intermediate area of the low-pressure column 15. Theraw liquid oxygen 29 is conducted out of the high-pressure column 11, inthis case through the pipes 129 (systems with argon), into the argonfraction and is partially vaporized, particularly at least in part inthe top condenser of the raw-argon column(s) (not depicted). The atleast partially vaporized raw oxygen is fed in through piping 103 intothe low-pressure column 15, which remains liquid through piping 132.Likewise, a gaseous residue stream (waste) 104 is drawn off from theargon fraction 100.

Alternatively, or in addition to the internally compressed productsdescribed for the first version, the pure liquid argon 105 can be passedon for internal compression, in which it is brought as a third “liquidproduct stream” into an argon pump 106 at a third increased pressurethat corresponds to the desired product pressure and which must not bethe same as the first and/or second increased pressure. Thehigh-pressure argon is conducted through piping 107 to the cold end ofthe main heat exchanger 9. In the main heat exchanger 9, the argonstream 107 is vaporized or pseudo-vaporized under the increased pressureand is heated up to roughly ambient temperature. It finally leaves thesystem through piping 108 as a third gaseous compressed product(HP-GAR).

The main heat exchanger can be executed as integral or split. Thedrawings show only the function of the exchanger: hot streams are cooledto cold.

FIG. 2 corresponds in large part to FIG. 1. Hence the same referencesare used as for the procedural steps and apparatus parts alreadydescribed above, and the air compressor, the air purification, and thedistillation-column system are not depicted in FIG. 2.

A difference from FIG. 1 is the higher outlet pressure of the aircompressor (“first pressure”), which in FIG. 2 clearly lies above theoperating pressure of the high-pressure column and amounts to 17 bars inthe actual example. On this basis, the direct air stream (27 in FIG. 1)is also missing. Rather, the total air 8 is divided downstream of therecompressor 7 under roughly 22 bars (“second pressure”) at 203 into theturbine stream 17 and the throttling stream 11. (In FIG. 2, the cooldownof the turbine and throttling streams could also be executed together,whereby the fractionation within the main heat exchanger 9 could beexecuted just in front of its cold end). The temperature of the turbinestream 17 in the example, before throttling 18, is 1 K to 50 K above thetemperature of the cold end and is also above the temperature at whichthe throttling stream 11 leaves the main heat exchanger. Alternatively,the turbine stream could also, as shown in FIG. 1, be passed on as faras the cold end of the main heat exchanger 9.

In addition, the aftercooler 202 of the recompressor is represented inFIG. 2, which may also used in the process according to FIG. 1 but isnot shown. The reference number 201 indicates the optional cooldown ofthe main air stream 5 downstream of the recompressor 7 in the main heatexchanger 9.

FIG. 3 is distinguished from FIG. 1 by a second expansion machine 319, arecompressor 304, and an aftercooler 305. The branch-off of turbinestream and throttling stream takes place here in heat at 303, wherebythe throttling stream in the second recompressor 304 with the secondpressure (here, for example, 22 bars) is further compressed to a thirdpressure (here, for instance 25 bars). The aftercooler 302 behind thefirst recompressor 7 can be omitted if the pre-cooling 201 of the mainair stream is used.

The process stream 270, with which the second expansion machine isoperated, can be formed by one of the following streams:

-   -   A partial stream of the turbine stream 70 (in this case, the        expanded stream 325 is mixed with the stream 25 from the first        expansion machine (both expansion machines in parallel).    -   A further air stream, which under the inlet pressure of the        first recompressor 7 or under the outlet pressure of the first 7        or second 304 recompressors is taken off at an intermediate        temperature from the main heat exchanger and is fed downstream        of the expansion machine 319 into the low-pressure column or        into the high-pressure column (15 and 14 in FIG. 1) (Lachmann or        a second Claude turbine).    -   A compressed nitrogen stream (35, 64, 67 in FIG. 1 or a partial        stream respectively thereof) out of the high-pressure column or        the low-pressure column.

Alternatively, the coupling between the turbines 19, 319 and therecompressors 7, 304 is also the converse of that depicted in FIG. 3.

FIG. 4 is thereby distinguished from FIG. 3, in that the secondrecompressor 403, which further compresses only the throttling stream,is constructed as a cold compressor.

In FIGS. 5 (for prior art) and 6 (for FIG. 2), the effect of throttlingaccording to the invention downstream of the expansion machine is readoff on the H-T diagram of the main heat exchanger. In FIG. 6, theturbine inlet temperature (stream 70 in FIG. 2; Tin) is clearly lowerthan in FIG. 5. The curves for the cooled-down streams (above) and theheated-up streams (below) lie considerably closer to one another. Theexchange losses are accordingly smaller.

What is claimed is:
 1. A process for the low-temperature separation ofair with a distillation-column system comprising at least one separationcolumn, said process comprising: compressing a main air stream in an aircompressor at a first pressure and purifying the main air stream in apurification device; compressing a first air stream, which is formedfrom at least one portion of the purified main air stream, in arecompressor at a second pressure that is higher than the firstpressure; branching off a throttling stream and a turbine steam from thefurther compressed first air stream; cooling the throttling stream andliquefying or pseudo-liquefying the throttling stream in a main heatexchanger and passing the throttling stream to an expansion equipment;conducting the expanded throttling stream into the distillation-columnsystem; cooling the turbine stream in the main heat exchanger and, underan intermediate temperature of the main heat exchanger, conducting theturbine stream into an expansion machine and expanding the turbinestream to produce work; conducting the expanded turbine stream at leastin part into the distillation-column system; taking off a liquid productstream from the distillation-column system at an increased pressure andvaporizing or pseudo-vaporizing the liquid product stream by indirectheat exchange with the throttling stream and drawing off as a gaseousproduct stream, wherein the turbine stream is cooled so that, if thepressure of the turbine stream is subcritical, the dewpoint temperatureor a lower temperature is attained, or, if the pressure of the turbinestream is subcritical, the turbine stream is pseudo-liquefied, whereinthe cooled-down or pseudo-liquefied turbine stream is expanded at leastat the dewpoint temperature at an intermediate pressure that is belowthe second pressure, and wherein the turbine stream expanded at theintermediate pressure is heated up in the main heat exchanger at theintermediate temperature before it is conducted to an expansion machine.2. A process according to claim 1, wherein the recompressor is driven byexternal power and both the throttling stream and the turbine streamupon cooldown in the main heat exchanger are under the second pressure,where the intermediate pressure is in between the first and the secondpressure.
 3. A process according to claim 1, wherein the recompressor isdriven by an expansion machine, which is operated with a process stream,in particular by the expansion machine that is operated with the turbinestream, in which the air compressor is the only machine driven byexternal power for the compression of air.
 4. A process according toclaim 3, wherein the recompressor is driven by an expansion machineoperated with the turbine stream.
 5. A process according to claim 1,wherein the turbine stream upon cooldown in the main heat exchanger isunder the second pressure and the throttling stream upon cooldown in themain heat exchanger is under a third pressure that is the same as thesecond pressure or is higher than the second pressure.
 6. A processaccording to claim 1, wherein the intermediate pressure is 1.5 to 5 barsbelow the second pressure.
 7. A process according to claim 1, whereinthe distillation-column system comprises a high-pressure column and alow-pressure column located above a main condenser.
 8. A processaccording to claim 7, wherein a liquid oxygen stream is taken off fromthe low-pressure column or the main condenser and is used as a liquidproduct stream.
 9. A process according to claim 7, wherein a liquidnitrogen stream is taken off from the high-pressure column or the maincondenser and is used as a liquid product stream.
 10. A processaccording to claim 1, wherein a liquid argon stream is taken off andused as a liquid product stream.
 11. A process according to claim 1,wherein a second air stream is formed from a portion of the purifiedmain air stream and the second air stream is cooled down under the firstpressure in the main heat exchanger and is conducted to thedistillation-column system.
 12. A device for the low-temperatureseparation of air with a distillation-column system having at least oneseparation column, comprising: an air compressor for compressing a mainair stream at a first pressure; a purification device for purifying themain air stream compressed at the first pressure; a recompressor forcompressing a first air stream, which is formed from at least oneportion of the purified main air stream, at a second pressure that ishigher than the first pressure; means for branching off a throttlingstream and a turbine stream from the cooled, further compressed firstair stream; a main heat exchanger for cooldown of the throttling streamand the turbine stream; means for liquefying or pseudo-liquefying thethrottling stream in the main heat exchanger; expansion equipment forexpanding the liquefied or pseudo-liquefied throttling stream; means forconducting the expanded throttling stream into the distillation-columnsystem; means for conducting the turbine stream under an intermediatetemperature of the main heat exchanger into an expansion machine forexpansion of the turbine stream to produce work; means for conductingthe turbine stream, expanded to produce work, into thedistillation-column system; means for offtake of a liquid product streamfrom the distillation-column system, for an increase in its pressure ina liquid state to an increased pressure, and for vaporization under thisincreased pressure by indirect heat exchange with the throttling stream;means for drawing off the (pseudo-)vaporized product stream as a gaseousproduct stream; means for drawing off the turbine stream from the mainheat exchanger at a position at which, in the operation of the device,if the pressure of the turbine stream is subcritical, roughly thedewpoint temperature of the turbine stream or a lower temperature isattained, or if the pressure of the turbine stream is subcritical, theturbine stream is pseudo-liquefied; means for expanding the turbinestream drawn off from the main heat exchanger at an intermediatepressure that lies below the second pressure; and means of heating upthe turbine stream expanded at the intermediate pressure, in the mainheat exchanger to the intermediate temperature, in which said means isdownstream of the expansion machine.
 13. A process for thelow-temperature separation of air, comprising: compressing a main airstream to a first pressure; purifying the main air stream; forming afirst air stream from at least one portion of the purified main airstream; compressing the first air stream to a second pressure that ishigher than the first pressure; branching off a throttling stream and aturbine steam from the compressed first air stream; liquefying orpseudo-liquefying the throttling stream in a main heat exchanger;cooling the turbine stream in the main heat exchanger so far that, ifthe pressure of the turbine stream is subcritical, the dewpointtemperature or a lower temperature is attained, or, if the pressure ofthe turbine stream is subcritical, the turbine stream ispseudo-liquefied; expanding the cooled or pseudo-liquefied turbinestream to an intermediate pressure that is below the second pressure;heating the turbine stream expanded at the intermediate pressure in themain heat exchanger is heated up at the intermediate temperature;conducting the heated expanded turbine stream at the intermediatepressure to an expansion machine and conducting at least a part of thefurther expanded turbine stream to a distillation-column system; andtaking a liquid product stream off from the distillation-column system,bringing the liquid product stream to an increased pressure, vaporizingor pseudo-vaporizing the liquid product stream by indirect heat exchangewith the throttling stream and drawn off as a gaseous product stream.14. A device for the low-temperature separation of air, comprising: anair compressor for compressing a main air stream; a purification devicefor purifying the compressed main air stream; a recompressor forcompressing a first air stream, formed from at least one portion of thepurified main air stream; a line for branching off a throttling streamand a turbine stream from the compressed first air stream; a main heatexchanger for cooling the throttling stream and the turbine stream,thereby liquefying or pseudo-liquefying the throttling stream; anexpansion equipment for expanding the liquefied or pseudo-liquefiedthrottling stream; a line for conducting the expanded throttling streaminto a distillation-column system; a line for drawing off the turbinestream from the main heat exchanger; an expansion machine for expandingthe turbine stream drawn off from the main heat exchanger to anintermediate pressure; a line for conducting the expanded turbine streamat intermediate pressure to the main heat exchanger to heat to anintermediate temperature; a line for conducting the turbine stream underan intermediate temperature to an expansion machine; a line forconducting the expanded turbine stream into the distillation-columnsystem; a line for conducting a liquid product stream from thedistillation-column system, and having a pump to increase its pressurein a liquid state, and for vaporization in the main heat exchanger withthe throttling stream; and a line for drawing off the (pseudo-)vaporizedproduct stream as a gaseous product stream.